U.S. patent number 6,625,371 [Application Number 09/959,322] was granted by the patent office on 2003-09-23 for planar optical waveguides with double grooves.
This patent grant is currently assigned to British Telecommunications public limited company. Invention is credited to Graeme D Maxwell, Alistair J Poustie, David C Rogers.
United States Patent |
6,625,371 |
Rogers , et al. |
September 23, 2003 |
Planar optical waveguides with double grooves
Abstract
A planar waveguiding device includes at least on section of core
which is located between and adjacent to two grooves. The
refractive index within the grooves is substantially equal to one
and the grooves are located so that the evanescent fields of
optical signals travelling in the core extend into the grooves.
Preferably the grooves have a direct interface with the core and
they extend through a layer located above the core into a layer
located below the core. Where the cores have bends, e.g. bends with
radii of curvature below 2 mm the grooves are located both inside
and outside the bends.
Inventors: |
Rogers; David C (Ipswich,
GB), Maxwell; Graeme D (Ipswich, GB),
Poustie; Alistair J (Ipswich, GB) |
Assignee: |
British Telecommunications public
limited company (London, GB)
|
Family
ID: |
27443843 |
Appl.
No.: |
09/959,322 |
Filed: |
October 23, 2001 |
PCT
Filed: |
May 19, 2000 |
PCT No.: |
PCT/GB00/01916 |
PCT
Pub. No.: |
WO00/72061 |
PCT
Pub. Date: |
November 30, 2000 |
Foreign Application Priority Data
|
|
|
|
|
May 21, 1999 [EP] |
|
|
99303961 |
May 21, 1999 [EP] |
|
|
99303962 |
May 21, 1999 [EP] |
|
|
99303963 |
May 21, 1999 [EP] |
|
|
99303964 |
|
Current U.S.
Class: |
385/132 |
Current CPC
Class: |
G02B
6/12011 (20130101); C03C 15/00 (20130101); G02B
6/125 (20130101); G02B 6/132 (20130101); G02F
1/225 (20130101); G02B 6/136 (20130101); G02B
6/13 (20130101); G02B 2006/12159 (20130101); G02F
1/0113 (20210101); G02F 1/0147 (20130101); G02B
2006/12145 (20130101) |
Current International
Class: |
C03C
15/00 (20060101); G02B 6/34 (20060101); G02B
6/132 (20060101); G02B 6/125 (20060101); G02B
6/13 (20060101); G02F 1/225 (20060101); G02B
6/136 (20060101); G02F 1/01 (20060101); G02B
6/12 (20060101); G02B 006/10 () |
Field of
Search: |
;385/129-132,147 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
0285351 |
|
Oct 1988 |
|
EP |
|
0297851 |
|
Jan 1989 |
|
EP |
|
0583903 |
|
Feb 1994 |
|
EP |
|
0793122 |
|
Sep 1997 |
|
EP |
|
WO 94 16345 |
|
Jul 1994 |
|
WO |
|
Other References
Patent Abstracts of Japan, vol. 1998, No. 09, Jul. 31, 1998, &
JP 10 090543A, Apr. 10, 1998. .
Patent Abstracts of Japan, vol. 1997, No. 03, Mar. 31, 1997 &
JP 08 304644 A, Nov. 22, 1996. .
Li et al, "Silica-Based Optical Integrated Circuits", IEE
Proceedings: Optoelectronics, GB, Instititution of
ELectricalengineers, Stevenage, vol. 143, No. 5, Oct. 1, 1996, pp.
263-280. .
Seo et al, "Low Transition Losses in Bent Rib Waveguides", Journal
of Lightwave Technology, US, IEEE, New York, vol. 14, No. 10, Oct.
1, 1996, pp. 2255-2259. .
Spiekman et al, "Ultrasmall Waveguide Bends: The corner Mirrors of
the Future?" IEE Proceedings; optoelectronics, vol. 142, No. 1,
Feb. 1, 1995, pp. 61-65. .
Chulhun Seo et al, "Optical Bent Rib Waveguide With Reduced
Transition Losses", IEEE Transactions on Magnetics, vol. 32, No. 3,
May 1996, pp. 930-933. .
Suzuki et al, "High-Density Integrated Planar Lightwave Circuts
Using Sl02-GE02 Waveguides with a High Refractive Indec
Difference", Journal of Lightwave Technology, vol. 121, No. 5, May
1, 1994, pp. 790-796. .
Himeno et al, "Loss Measurement and Analysis of High-Silica
Reflection Bending Optical Waveguides", Journal of Lightwave
Technology, vol. 6, No. 1, Jan. 1, 1988, pp. 41-46..
|
Primary Examiner: Ullah; Akm Enayet
Attorney, Agent or Firm: Nixon & Vanderhye P.C.
Claims
What is claimed is:
1. A planar waveguiding device which comprises a core and a
cladding, said core having a higher refractive index than said
cladding, wherein the device includes at least one section of core
which is located between and adjacent to two grooves wherein the
refractive index within the grooves is substantially equal to one,
the location of the grooves being such that the evanescent fields
of optical signals travelling in the core extend into the grooves,
said core and said cladding being formed of amorphous material.
2. A planar waveguiding device according to claim 1, in which the
grooves have a direct interface with the core.
3. A planar waveguiding device according to claim 1, in which the
grooves extend through a layer located above the core into a layer
located below the core.
4. A planar waveguiding device according to claim 1, in which
grooves are located inside and outside a bend portion of core, said
bend portion having a radius of curvature less than 2 mm.
5. A planar waveguiding device according to claim 4, wherein the
radius of curvature is less than 500 .mu.m.
6. A planar waveguiding device according to claim 4, wherein the
core includes a plurality of bends, each comprising a portion
having a radius of curvature less than 2 mm, wherein the device
comprises grooves located inside and outside each bend, each of
said grooves having an interface with the core.
7. A planar waveguiding device according to claim 6, wherein at
least one of the bends has a portion with a radii of curvature of
less than 500 .mu.m.
8. A planar waveguiding device according to claim 4, wherein the
core comprises straight segments interconnected by curves having a
radius of curvature less than 2 mm.
9. A planar waveguiding device according to claim 4, wherein the
core comprises separate portions which are not interconnected.
10. A planar waveguiding device according to claim 4, wherein the
core comprises a junction or junctions where a plurality of paths
converge into a single path and/or a divergence or divergences
where a single path divides into a plurality of paths, said
junctions and/or divergences including curves having a radius of
curvature less than 2 mm.
11. A planar waveguiding device according to claim 1, wherein the
amorphous material of the cladding is a cladding glass and the
amorphous material of the core is a core glass having a higher
refractive index than the cladding glass.
12. A planar waveguiding device according to claim 11, wherein the
core glass and the cladding glass both contain at least 90 wt % of
silica.
13. A planar waveguiding device according to claim 12, wherein the
core and the cladding are supported on a silicon substrate.
14. A planar waveguiding device according to claim 12, wherein the
core glass is a mixture of SiO.sub.2 and GeO.sub.2 and the cladding
glass comprises a buffer region of pure SiO.sub.2 a covering region
of SiO.sub.2 doped with processing aids selected to give a
refractive index equal to that of pure SiO.sub.2.
15. A planar waveguiding device according to claim 1, wherein the
grooves extend at least 5 .mu.m from the core.
16. A planar waveguiding device according to claim 1, wherein the
grooves extend at least 30 .mu.m from the core.
17. A planar waveguiding device according to claim 1, wherein the
grooves are open to ambient atmosphere whereby the grooves contain
ambient atmosphere.
18. A planar waveguiding device according to claim 1, wherein
.DELTA.n=substantially 0.01.
Description
This invention relates to planar optical waveguides and, in
particular, to planar optical waveguides which include bends.
Optical waveguides exist in two configurations, namely fibre and
planar. The planar configuration is convenient for the processing
of optical signals and the term "planar" is used because the path
regions are located in an essentially two-dimensional space. The
path regions are formed of an amorphous material and they are
enclosed in a matrix of one or more different amorphous materials
ideally having the same refractive index as one another. The
refractive index of the matrix is less than the refractive index of
the material forming the path regions. The difference between the
two refractive indices is often represented by .DELTA.n and for the
condition for effective guidance with low attenuation is usually
.DELTA.n=0.01 (approximately).
The amorphous materials are preferably glass, e.g. silica based
glass. Silica doped with germania is particularly suitable for the
path regions. In the case of the matrix pure silica or silica
containing processing aids such as oxides of phosphorus and/or
boron are particularly suitable. (Pure silica has a. refractive
index of 1.446 and this is a convenient refractive index for the
whole of the matrix. Germania increases the refractive index of a
silica glass.)
Although, as mentioned above, planar waveguiding structures are not
fibre, the term "core" is often used to denote the path regions and
the matrix in which the cores are embedded is often called the
"cladding".
The condition stated above is appropriate for most of a waveguide
but this invention relates to special portions where different
considerations apply. According to this invention a planar
waveguiding device includes regions wherein a segment of core is
located adjacent to a groove or between two grooves. Preferably the
groove or grooves extend above and below said segment of core. It
is desirable that the evanescent fields of signals travelling in
the core penetrate into the groove.
The maximum extent of the evanescent fields outside the core is
usually less than 1 .mu.m and therefore any coating between the
core and the groove should be less than 500 nm. Preferably there is
a direct interface between the core and the groove. Localised
heating of cores offers one way of causing localised changes of
refractive index, e.g. for Max Zender devices. A heating element
can be located on top of the core adjacent to one or two grooves.
The grooves restrict the transmission of heat.
In some applications material may be located in the groove, e.g.
for use as a sensor or for testing the material in the groove. In
these applications the material is placed in the groove after the
device has been made, e.g. material is placed in the groove and ,
if necessary, replaced in accordance with requirements.
Usually the purpose of the groove is to provide a very low
refractive index adjacent to the core, i.e. to make .DELTA.n as big
as possible. The lowest refractive index, namely 1, is provided by
an empty groove (i.e. vacuum) but most gases also have a refractive
index substantially equal to one. "Empty" grooves as described
above are particularly valuable where cores pass round bends. This
is a preferred embodiment of the invention and it will be described
in greater detail below.
A high proportion of the cores consists of straight lines but
possible uses are severely limited if the cores consist only of
straight lines and, in general, signal processing is not possible
in planar devices wherein the cores consist only of straight lines.
Many planar devices include multiplexers and/or demultiplexers and
curves are needed to form these. Curves are also needed if it is
desired to create a serpentine path in order to increase its
length, e.g. for a laser. Complicated devices, such as arrayed
waveguide gratings (AWG), require many bends.
In many devices the radius of curvature of the bend is a critical
parameter in determining the overall size of the device. For
example, a small radius of curvature will place waveguide segments
close together whereas a large radius of curvature will cause the
segments to be more widely separated. In order to provide more
processing capability on the same size of wafer it is desirable to
make the devices as small as possible and, since the radius of
curvature is a critical parameter, it is desirable to make the
radius of curvature as small as possible. In some cases, the
spacing of waveguides on a wafer is determined by external
constraints and it may be necessary to use a small radius of
curvature in order to conform to the external constraints.
It will be appreciated that a curved path may be a circle or a
segment of a circle and in such a case the radius of curvature of
the path is constant, i.e. it is equal to the radius of the circle.
If a curved path is not circular it will still have a radius of
curvature but this radius will vary from point to point along the
curve. Nevertheless, it is still true that a small radius of
curvature will favour closer packing of devices. It is usually
convenient to measure the radius of curvature to the centre of the
core but there will be significant differences between the inside
and the outside of the curve.
The guidance of optical radiation round shallow bends, e.g. with
radii of curvature of 5 mm or more does not cause problems but
sharp bends, e.g. with radii of curvature below about 2 mm, can
cause noticeable degradation of performance. These problems can
become severe when it is desired to use even smaller radii of
curvature, e.g. less than 500 .mu.m.
According to a preferred embodiment of this invention, a planar
waveguiding device comprises a core having a bend with an inner
radius of curvature and an outer radius said inner radius of
curvature being less than 2 mm wherein "empty" grooves are located
adjacent to both said inner and said outer radii of curvature, said
grooves preferably having an interface with the core and extending
both above and below the core. Since the grooves are prepared by
etching they will normally extend to the surface of the device but
it is desirable to continue the etching below the bottom of the
core in order to improve the guidance. It has been stated that the
grooves are "empty". Conveniently, the grooves are allowed to
contain whatever atmosphere is present where the device is used. In
most cases, the atmosphere will be air but, in space there would be
a vacuum. The refractive index in the groove is substantially equal
to one because this is the refractive index of a vacuum and
virtually all gasses have a refractive index equal to one.
In one aspect, this invention is concerned with the problem of loss
of guidance at bends which may result in the radiation escaping
from the core. The severity of this problem is strongly related to
the radius of curvature of the bend and the smaller the radius of
curvature the worse the problem. Where the radius of curvature is
above 5 mm there is no problem but there is a substantial problem
when the radius of curvature is 2 mm or less. The problem gets even
worse at smaller radii of curvature, e.g. below 500 .mu.m. The
location of grooves will give usefully low attenuation at radii of
curvature down to about 50 .mu.m. It will be appreciated that some
waveguiding structures will include a plurality of bends. There
would be no advantage in providing grooves adjacent to curves with
radii of curvature greater than 5 mm and it is highly desirable
that all bends with radii of curvature less than 2 mm, and
especially less than 500 .mu.m, are provided with grooves in
accordance with the invention.
The electric and magnetic fields associated with light propagating
in the cores extend outside the cores and, ideally, the groove
should be so located and sufficiently wide that these fields are
contained entirely in the grooves. For wavelengths of the order 1.5
.mu.m the fields extend for about 1 .mu.m beyond the core. For most
purposes, grooves which are 30 .mu.m wide will be sufficient. There
is no objection to using greater widths where these are convenient
and compatible with the overall structure.
Waveguiding devices in accordance with the invention can be
manufactured using conventional fabrication techniques. For
example, it is convenient to deposit a sequence of glass layers by
flame hydrolysis using conventional photolithography to produce
path regions and grooves. In order that there is an interface
between the groove and the core, it is appropriate to etch the core
to extend beyond the boundaries of the curve and to remove core
material when the groove is etched. Reactive ion etching is
particularly suitable for producing the grooves because this
technique is inherently monodirectional and it produces grooves
with vertical sides.
The invention will now be described by way of example with
reference to the company drawings in which:
FIG. 1 is a plan view illustrating the location of grooves for a
90.degree. bend;
FIG. 2 is a cross section on the radial line AA of FIG. 1;
FIG. 3 corresponds to FIG. 1 but illustrating the configuration
before the etching of the groove; and
FIG. 4 corresponds to FIG. 2 illustrating the configuration during
the etching process.
FIG. 5 illustrates the configuration of an arrayed waveguide
grating (AWG).
FIG. 6 illustrates the configuration of waveguides and grooves
comprised in the AWG of FIG. 5.
FIG. 7 illustrates the grooves comprised in the AWG of FIGS. 5 and
6.
FIG. 8 illustrates the tapers at the ends of grooves as shown in
FIG. 7.
FIG. 9 illustrates a Max Zender device with grooves to enhance
thermal control, and.
FIG. 10 is cross section through the Max Zender device of FIG.
9.
FIG. 1 illustrates a core 10 which includes a bend through
90.degree.. In accordance with the invention there is an empty
groove 11 on the outside of the bend and an empty groove 12 on the
inside of the bend.
The refractive index within both grooves is substantially equal to
one, e.g. both contain air. (All the refractive indexes quoted in
these examples were measured using radiation with a wavelength of
1523 .mu.m).
The core 10 had a square cross section and the sides of the square
were 10 .mu.m. The bend is a quadrant of a circle and the radius of
the circle (measured to the central line of the core 10) is 125
.mu.m. The outer wall 13 of the groove 11 is also the quadrant of a
circle but in this case the circle has a radius of 160 .mu.m.
Similarly, inner wall 14 of the groove 12 is also the quadrant of a
circle but in this case the circle has a radius of 90 .mu.m. From
these dimensions, it will be appreciated that each of the grooves
11 and 12 is 30 .mu.m wide.
FIG. 2 shows a vertical cross section along the line AA at FIG. 1.
This is a radial cross section and it is substantially identical
along any radius of the bend.
FIG. 2 shows the conventional layers of planar waveguiding devices
and these layers comprise, from the upper surface downwards:
Covering layer 21 which is formed of silica with processing agents;
Cores 10; Buffer layer 23 (and optionally 22) which is formed of
pure silica (without any additives); and The silicon substrate
24.
(The silicon substrate 24 provides mechanical support for the
structure but it may not contribute to the optical function.
Usually, the buffer layer 22, 23 is sufficiently thick that the
fields associated with optical signals do not penetrate into the
silicon substrate 24).
As is conventional for the preparation of glass planar waveguides
devices, the starting point was a substrate (which is purchased
commercially). The commercial substrate comprised a layer 24 of
silicon and the surface of this silicon wafer was oxidised to
produce an adherent thin layer 23 of silica (which is part of the
buffer layer 22, 23 between the core 10 and the silicon layer
24).
As a first stage of preparation a uniform buffer layer of pure
silica was deposited by flame hydrolysis and the residue of this
layer is indicated by 22. The core 10 was deposited, originally as
a uniform layer on the buffer layer 22 (if desired, the deposited
layer 22 can be omitted and the core 10 deposited directly upon the
thin layer 23 of silica). This layer was also deposited by flame
hydrolysis but GeCI.sub.4 was introduced to the flame to produce a
layer of silica doped with germania to increase the refractive
index of the silica to 1.456. After deposition, the unwanted
portions of this layer were removed by conventional
photolithography to produce the core 10.
After etching, the whole area was covered by a covering layer 21 of
silica by flame hydrolysis and both boron and phosphorous were
introduced into this layer to reduce the melting point. The ratio
of the boron and phosphorous was adjusted so that the layer 21 has
the same refractive index as pure silica, namely 1.446. Originally,
the layer 21 was deposited as a fine soot which was melted to give
a compact layer 21 which fills all the spaces between the etched
core 10. This normally completes the preparation of a planar
waveguiding device but, in accordance with the invention, the
grooves 11 and 12 were etched. As can be seen from FIG. 2 the
grooves 11 and 12 extend completely through the covering layer 21
and into the buffer layer 22. Thus there are interfaces 15 and 16
between the core 10 and the grooves 11 and 12.
The grooves 11 and 12 can be regarded as "empty" because no filling
is placed therein. However, any atmosphere in which the device is
located will penetrate into the grooves. The atmosphere is gaseous
and, in most circumstances, the atmosphere will be air. If the
device were used in a spacecraft it is possible that the grooves
would contain vacuum. However, the refractive index in the groove
is substantially equal to one because this refractive index applies
to both vacuum and gasses. The configuration illustrated in FIGS. 1
and 2 has the effect that, at the bend, any fields which extend
into the grooves 11, 12 will be located in a region which has a
refractive index of one. This has two major effects which will now
be described.
The core 10 has a refractive index of approximately 1.456 so that
the difference in refractive index between the core 10 and the
grooves 11 and 12 is 0.456. This is a very high difference and it
gives very strong guidance whereby radiate losses are reduced at
the bend and satisfactory guidance round the bend is achieved.
However, the interfaces 15 and 16 represent boundaries associated
with a high refractive index difference and, therefore, there is
substantial loss by scattering from the interfaces 15 and 16. These
high scattering losses would not be tolerable over substantial path
lengths but the bends only account for a small proportion of the
path length and, therefore, high scattering does not result in
substantial overall loss. Furthermore, the bend has a small radius
of curvature (since the invention is particularly concerned with
bends having a small radius of curvature) and, therefore, the
circumferential distance around the bend is also small. For
example, the distance around the bend illustrated in FIG. 1 (based
on the centre of the core 10) is approximately 200 .mu.m. The
height of the core 10 is 10 .mu.m so that the total area of two
interfaces 15, 16 is small, approximately 4000 (.mu.m).sup.2.
As mentioned above, the method of producing a planar waveguiding
structure is substantially conventional. However, the method of
producing the interfaces 15 and 16 will now be described in greater
detail.
FIG. 3 indicates the configuration at the bend immediately before
the production of the grooves 11 and 12. When the core 10 was
etched a very wide core 30 was left at the bend. As a preparation
for etching the grooves 11 and 12, the surface of the device is
covered with a mask which leaves apertures over the intended
grooves 11 and 12. The grooves 11 and 12 are produced by reactive
ion etching which technique is highly directional normal to the
surface of the device. This produces grooves with vertical walls
but the location of the grooves is controlled by the mask. Thus,
the etching removes the material in the grooves including the
excess material in the path region 30.
FIG. 4 is a cross section on the line AA of FIG. 3. It illustrates
the configuration produced near the end of the etching. Part of the
interfaces 15 and 16 have already been produced but the expanded
core 30 has horizontal surfaces 31 and 32 which are exposed to the
etching. As the etching proceeds, the surfaces 31 and 32 are eroded
until, at the end of the etching, all of the excess 30 has been
removed. It will be appreciated that this technique produces the
interfaces 15 and 16 during etching and it ensures that these two
surfaces form a boundary between core having a refractive index
approximately 1.5 and a groove space having a refractive index
substantially equal to one. The effect of this arrangement has
already been explained.
Arrayed waveguide gratings (AWG) have several uses in the
processing of optical signals. AWG require many, at least 25,
usually 50 to 500 and typically about 150 separate paths whereby
gratings effects are produced by interference between radiation
travelling in different paths. The paths include changes of
direction and, for reasons which will be explained later, it is
desirable to provide the changes of direction by tight bends, e.g.
bends having radii of curvature less than 150 .mu.m . The structure
of such AWG will now be described with reference to FIGS. 5, 6 and
7.
FIG. 5 provides a highly diagrammatic representation of an AWG. The
important components of an AWG are a grating region 51 which is
shown in greater detail in FIGS. 6 and 7. In order to make external
connections the AGW includes input/output IO regions 52a and 52b.
Since the paths of light are usually reversible, it is convenient
for the input/output IO regions 52a and 52b to be symmetrical, e.g.
of identical construction.
Each of the IO regions 52a, 52b comprises an I/O slab 53a, 53b and
connector paths 54a, 54b. Each of the IO slabs 53a, 53b is a large
region having a uniform refractive index equal to that of the path
regions 10. Each I/O slab 53 has curved boundaries, one of which
engages with the connectors 54 and the other of which engages with
the paths 61, 62, 63 comprised in the grating region 51. It is the
function of a IO slab 53 to distribute radiation received on any
one of the connectors 54 uniformly into the plurality of paths
comprised in the grating region 51.
FIG. 6 shows the general layout of the grating region 51. As
mentioned above, this region comprises plurality of paths but, for
ease of illustration, only three paths are illustrated. These are
the inner path 61, the outer path 62 and a typical path 63. (The
typical path 63 is repeated many times.) The paths have two changes
of direction located along the lines 64a and 64b. The effect of
these changes of direction is that the path 61 is the shortest and
path 62 is the longest. As can be seen from FIG. 6, the paths
follow a circuitous route around an approximate centre 65. The
paths are graded in length depending upon the distance from the
centre 65.
If the length of the shortest path 61 is designated by L .mu.m
then, ideally, the other paths should have lengths of
where n is the total number of paths.
It is the purpose of the grating region 51 to produce interference
effects by reason of phase changes produced in the various paths.
Therefore .DELTA.L is the critical parameter and it is important
that .DELTA.L shall be constant between any two adjacent paths.
Since interference effects are dependent upon fractions of a
wavelength (which is typically of the order 1.5 .mu.m ), .DELTA.L
must be very accurate. This imposes the requirement that the total
length of the paths must be determined to the same accuracy. The
measured length of the path, i.e. the length in micrometers, can be
accurately fixed from the photolithography but the effective length
of the path is dependent upon other considerations. Since the
refractive index controls the speed of propagation of light in the
path, it is important that the refractive index, and hence the
chemical composition, shall be uniform over the whole of the
grating region 51 and this is difficult to achieve with a large
region. Furthermore, the irregularities in the cross sectional area
of the paths can also effect speeds of propagation. In other words,
the uniformity of .DELTA.L is substantially affected by accurate
control of process variables and especially of the chemical
composition of the path regions. It is much easier to maintain
uniformity over a small region and, therefore, there is a strong
incentive to make the grating region 51 as small as possible.
It will be apparent from FIG. 6 that, in order to keep the size of
the grating region 51 as small as possible, it is necessary to keep
all the path regions as close as possible to the centre 65. For the
lengths to be as short as possible, the shortest path 61 must be as
short as possible and it is clear that closeness to the centre 65
is important in keeping the path as short as possible. When very
short path lengths are used, the configuration along the lines 64a
and 64b becomes important. It is not possible to have an abrupt
change of direction and, therefore, it is necessary to provide
smooth curves for all the paths. It is also necessary to keep the
radii of curvature of the path as small as possible. In order to
provide adequate guidance at the bends it is appropriate to provide
grooves 66 at the inside and outside of every bend. FIG. 6 does not
indicate the configuration of the grooves, it merely indicates
their location. The configuration of the grooves will be described
in greater detail with respect to FIG. 7.
FIG. 7 illustrates three adjacent paths 71, 72 and 73 at the bends.
Although only three paths are shown the same configuration occurs
at all bends for all paths. Path 71 has an inner groove 71.1 and an
outer groove 71.2 both of which have direct interfaces with the
path 71. The grooves 71.1 and 71.2 extend all the way round the
curve into the straight portions on both sides of the bend. There
is a region 74 of confining glass between the grooves 71.1 and 72.2
and a similar region 75 of confining glass between the grooves 71.2
and 73.1.
The grooves extend into the straight portions and therefore, the
ends of the grooves are in straight portions. To avoid sudden
transitions (which might adversely affect transition performance)
the grooves are preferably tapered as shown in FIG. 8 which
illustrates the edge 76 of a path, the edge 77 of confining glass,
and the taper 78. The taper rate (not in scale) is 50:1 to
100:1.
The Max Zender device shown in FIGS. 9 and 10 comprises a splitter
81 which divides an input 88 into a first path 82 and a second path
83. These converge at a junction 84 into an output 89. Changing
interference effects allows the arrangement to operate as a switch.
The first path 82 is located between grooves 85 and 86 and the
overlaying confinement 90 is covered with an actuator 87 which is
adapted to alter the refractive index of the underlying path 82.
The actuator preferably takes the form of an electric heating
element 87 (leads not shown). Heating the first path 82 (or a
suitable portion thereof) changes the length and refractive index
whereby the phase relations at the junction 84 are affected. The
grooves 85 and 86 localise the heating effect to give a faster
response time. As can be seen most clearly in FIG. 10, the paths
82, 83, 88 and 89 are supported on underlying layers 22, 23 and 24.
These layers are similar to those illustrated in FIGS. 2 and 4.
At the splitter 81 and the junction 84 it is appropriate to use
curves. Where the radii of the curves are small it is appropriate
to locate the curves between grooves as described earlier in this
specification.
* * * * *